[llvm-dev] RFC: Absolute or "fixed address" symbols as immediate operands

[Peter Collingbourne via llvm-dev]

On Mon, Oct 24, 2016 at 8:54 PM, Chandler Carruth <chandlerc at google.com>
wrote:

> On Mon, Oct 24, 2016 at 1:54 PM Peter Collingbourne via llvm-dev <
> llvm-dev at lists.llvm.org> wrote:
>
>> On Mon, Oct 24, 2016 at 1:36 PM, Friedman, Eli <efriedma at codeaurora.org>
>> wrote:
>>
>> On 10/24/2016 1:07 PM, Peter Collingbourne via llvm-dev wrote:
>>
>> On Mon, Oct 10, 2016 at 8:12 PM, Peter Collingbourne <peter at pcc.me.uk>
>> wrote:
>>
>> The specific change I have in mind is to allow !range metadata on
>> GlobalObjects. This would
>> be similar to existing !range metadata, but it would apply to the
>> "address" of the attached GlobalObject, rather than any value loaded from
>> it. Its presence on a GlobalObject would also imply that the address of the
>> GlobalObject is "fixed" at link time.
>>
>>
>> Going back to IR-level representation: here is an alternative
>> representation based on a suggestion from Eli.
>>
>> Introduce a new type of GlobalValue called GlobalConstant. GlobalConstant
>> would fit into the GlobalValue hierarchy like this:
>>
>>    - GlobalValue
>>    - GlobalConstant
>>       - GlobalPointer
>>          - GlobalIndirectSymbol
>>             - GlobalAlias
>>             - GlobalIFunc
>>          - GlobalObject
>>             - Function
>>             - GlobalVariable
>>
>> GlobalValue would no longer be assumed to be of pointer type. The
>> getType() overload that takes a PointerType, as well as getValueType()
>> would be moved down to GlobalPointer. (A nice side benefit of this is that
>> it would help flush out cases where we are unnecessarily depending on
>> global pointee types.)
>>
>> A GlobalConstant can either be a definition or a declaration. A
>> definition would look like this:
>>
>> @foo = globalconst i32 42
>>
>>
>> This is equivalent to writing "foo = 42" in assembly?
>>
>>
>> Yes.
>>
>>
>>
>>
>> while a declaration would look like this:
>>
>> @foo = external globalconst i32
>>
>> GlobalConstant could also hold a linkage and visibility. Looking at the
>> other attributes that a GlobalValue can hold, many of them do not seem
>> appropriate for GlobalConstant and could potentially be moved to
>> GlobalPointer.
>>
>> Thoughts?
>>
>>
>> How do you plan to use this?  The concept makes sense, but I've never
>> actually seen anyone use symbols this way.
>>
>>
>> I plan to use this as part of the ThinLTO implementation of control flow
>> integrity. See http://clang.llvm.org/docs/ControlFlowIntegrityDesign.html
>> for a description of how the design currently works in regular LTO.
>>
>> If you look at the asm snippets in that design doc, you will see a number
>> of hardcoded constants -- these constants are calculated at LTO time based
>> on whole program information. I want the intermediate object files to
>> depend on the constants so that their values can be substituted in at link
>> time. In ThinLTO, object files are cached, so if a value changes I want to
>> avoid invalidating the cache entries that depend on that value.
>>
>
> This states the context you want to use these in (CFI with ThinLTO)
> without actually stating how you plan to use them within that context. I
> think the latter would help motivate specific designs.
>
> Is there a write-up of the imagined CFI+ThinLTO design somewhere that
> (concisely) explains the plan?
>

Here it is:
http://lists.llvm.org/pipermail/llvm-dev/2016-October/106490.html

-- 
-- 
Peter
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20161025/f0330cd7/attachment.html>